The United States Government may have rights in the invention pursuant to funding arrangements with the Department of Defense.
1. Field of the Invention
This invention relates generally to flywheels and shafts, and more particularly to a fiber-reinforced composite flywheel or shaft, and a method for producing compressive prestresses in a fiber-reinforced composite flywheel or shaft. This invention may also be used to prestress other low tensile strength materials, for example ceramics, for use in a flywheel or shaft.
2. Description of the Related Art
In the field of electrical pulsed power generation, generators with increased power and energy storage capability are needed to satisfy a host of new applications. Laboratory electromagnetic accelerator experiments, such as impact fusion studies, require reliable high power sources, while other concepts, such as space launchers, may demand stored energies in the gigajoule range.
As described in Walls & Manifold, "Applications of Lightweight Composite Materials in Pulsed Rotating Electrical Generators" 6th IEEE Pulsed Power Conference, Arlington, Va (Jun. 29-Jul. 1, 1987), flywheels are attached to the rotors of pulsed power generators to increase energy storage capabilities. Flywheels function as reservoirs which store rotational kinetic energy. As energy is withdrawn from a spinning flywheel, its angular speed decreases; as energy is supplied to a spinning flywheel, its angular speed increases. Conventionally used steel flywheels, while improving the performance of generators, are limited due to their weight and relatively low maximum permissible tip speeds.
Composite materials, such as epoxy reinforced graphite fibers, have specific strengths about ten times greater than steel. Thus, energy storage flywheels made of hoop wound composite materials can be spun at higher tip speeds to achieve higher specific energy storage than steel flywheels.
The selection of a composite material for a pulsed power flywheel must take into account the magnetic field in the vicinity of the rotor as it spins. The use of a nonconductive composite flywheel and shaft reduces eddy current losses which could be generated as the magnetic field ramps up and down during excitation and discharge. These losses not only produce a drag torque on the rotor, but can also heat and damage the composite material. "KEVLAR," polyaramid fiber made by the Dupont Corporation, graphite, boron, or glass fiber are best suited for flywheel and shaft construction since they are relatively nonconductive.
When a nonconductive shaft and flywheel is used, it is also desirable to eliminate iron from the rotor of the generator completely and use an air-core magnetic circuit for exciting the rotor. This permits the flux density of the excitation field to be increased above the maximum level for iron core circuits, and when the increased flux density is coupled with the higher rotor speeds afforded by the composite flywheels, the generated voltage is substantially increased.
Composite materials, such as those mentioned above, exhibit phenomenal stiffness and strength in the axial direction of the fiber. These materials, however, are highly anisotropic. The transverse stiffness of a fiber composite can be thirty to forty times lower than the longitudinal stiffness, while the transverse strength can be two orders of magnitude lower than the longitudinal strength. Therefore, composite flywheels are typically constructed with the fibers wound predominantly in the circumferential direction (i.e., hoop wound). This construction results in less radial growth than a steel flywheel at any given speed. However, radial stress usually limits the flywheel's rotational speed due to the weak transverse strength of the composite material, and the severity of the problem usually increases with increasing flywheel thickness. Radial stresses produced by the rotation of the flywheel can cause the fibrous composite material to shred or crack circumferentially along the axis of the fibers.
To increase the maximum possible operating speeds of composite flywheels, they are usually constructed in an initial state of radial compression. As the flywheel's speed increases, the radial stress increases to zero, and then into the tensile region. Greater initial precompression leads to greater possible speeds, since the maximum tip speed is determined by the tip speed at which the radial stress exceeds the limit for the composite material.
Interference fits are commonly used to produce precompression. For example, two concentric annular sections of a flywheel have been assembled using an interference fit to produce radial compression in both sections. The interference fit is typically accomplished with tapered press fits rather than thermal fits, since fiber epoxy composites have coefficients of thermal expansion that are too low to provide a significant interface pressure. Interference fits produce a limited amount of precompression and satisfactory stress distribution. For tapered interference fits to be practical, however, the axial dimension of the annular sections must be relatively short to minimize axial assembly forces. Moreover, when a tapered interference fit is used, the assembly must be properly designed to guard against "growth mismatch" due to angular misalignment in the tapers. If not, the resulting uneven interface pressure could cause separation to occur at some portion of the interface, and such separation could lead to an unbalance which would damage the generator.
Fiber composite shafts present additional problems. A rotating shaft experiences radial, tangential, and bending stress. Therefore, more layers of fiber wound material are generally required to produce a shaft capable of performing well in the stressed condition. As stated previously, radial precompression produces flywheels having higher possible rotational speeds. Likewise, radial precompression produces shafts having higher terminal rotational speeds. Unfortunately, an interference fit which produces radial precompression between two annular sections of a shaft is impractical due to the long axial dimension of the shaft.